Chemical Strategies for Modifying Carbonyl/Nitrogen-Based MR-TADF Materials toward Narrowband Emission

The past decade has witnessed remarkable progress in multi-resonance thermally activated delayed fluorescence (MR-TADF) emitters based on nitrogen/carbonyl (N/C═O) frameworks, the quinolino[3,2,1-de]acridine-5,9-dione (QAO) derivatives, with a focus on achieving narrowband emission for high-performance OLED applications such as ultra-HD displays. This review categorizes and analyzes structural modifications across four key molecular architectures—pristine QAOs, phenyl-substituted QAOs, cyclized QAOs, and polynuclear QAOs—revealing their distinct impacts on full width at half maximum (FWHM), reorganization energy (λ), and excited-state dynamics. Notably, structural strategies such as R₁ substitution, R₄-R₅ cyclization, and steric shielding with bulky groups like tert-butyl lead to enh anced molecular rigidity, suppressed vibrational relaxation, and record-narrow emissions (FWHM ≤ 13 nm). Furthermore, donor-acceptor tuning across the series enables precise the highest occupied molecular orbital (HOMO) – the lowest unoccupied molecular orbital (LUMO) separation, balancing short-range charge transfer with emission color control. Emission statistics, fluorescence lifetime analysis, and device performance metrics are presented, consolidating structure-property relationships across >100 reported derivatives. This review provides design principles grounded in both theoretical insight and empirical evidence, offering a roadmap for the development of next-generation MR-TADF materials with high color purity, stability, and efficiency. These findings highlight the N/C = O MR core as a versatile and promising scaffold for advanced display technologies.